Updates 'Callbacks' section example to be correct
Updates 'Callbacks' section example to be correct

The current example is trying to define a before block inside of a
transition block, which is not a supported callback.  Moving the before
block in to the event block for an example that is supported.

pskrz/aasm,xieyunzi/aasm,ASnow/aasm,hspazio/aasm,nickpellant/aasm,allspiritseve/aasm,Aryk/aasm,joshsoftware/aasm,HoyaBoya/aasm,aasm/aasm,reidmorrison/aasm,artofhuman/aasm,Funfun/aasm